The present invention relates to the fabrication of semiconductor devices. More particularly, the present invention relates to improved techniques for fabricating electrically blowable fuses on a semiconductor substrate.
Fuses have long been employed in integrated circuits. A fuse typically comprises a fuse portion formed of a fuse material that may be turned into a non-conductive state through various mechanisms. When the fuse is in its conductive state, an electrical current may pass through the fuse portion. When the fuse is blown, i.e., becomes non-conductive, an open circuit is created through which very little, if any, current may pass.
Exemplary uses of fuses include, for example, protecting sensitive portions of the integrated circuit during manufacturing to prevent a build up of charge from damaging the sensitive electronic devices thereon. After the integrated circuit is manufactured, the fuse may be blown to sever the current path, and the resulting IC may be employed as if the current path never existed. Fuses may also be employed to, for example, set the address bits of a redundant element in a dynamic random access memory (DRAM) array in order to specify to the decoding circuit the address of the defective main memory array element. With the address information furnished by the fuses, the redundant element may then be employed to replace the defective main memory array element.
Although there are many fuse designs today, two types of fuses have received wide acceptance: laser blowable fuses and electrically blowable fuses. With laser blowable fuses, the fuses are typically formed at or near the surface of the integrated circuit. A laser beam striking the fuse material renders the fuse portion non-conductive, thereby inhibiting current from flowing through. Although laser blowable fuses are relatively simple to fabricate, there are disadvantages. For example, the laser blowable fuses tend to be surface oriented, which places a limitation on the design of the IC. Further, laser blowable fuses tend to occupy a large amount of space on the IC surface since the adjacent fuses or devices must not be placed too close to the fuse or risk being inadvertently damaged by the laser beam during the fuse setting operation.
Electrically blowable fuses, on the other hand, do not have to be placed at or near the surface of the integrated circuit. Accordingly, they give the designers greater latitude in fuse placement. In general, they tend to be smaller than laser blowable fuses, which render them highly suitable for use in modern high density integrated circuits.
In a typically electrically blowable fuse, the fuse portion, typically formed of a material that changes its state from conductive to non-conductive when a current exceeding a predefined threshold is passed through, is typically disposed in a dielectric microcavity, i.e., a sealed, hollow chamber in a dielectric layer. The microcavity itself is typically formed in a multistep process, which conventionally requires one or more photolithography steps in the prior art.
To facilitate discussion, FIGS. 1 and 2 illustrate the prior art process for forming an electrically blowable fuse. Referring initially to FIG. 1, a fuse portion 102 is shown disposed on a substrate 104. Fuse portion 102 typically comprises a conductor made of a suitable fuse material such as doped polysilicon or metal. For reasons which will become apparent shortly, the fuse portion is typically capped with a silicon nitride layer.
As mentioned, fuse portion 102 is dimensioned and configured such that when a current exceeding a predefined current value passes through fuse portion 102, it changes to a non-conductive state to essentially inhibit current from subsequently flowing through. Substrate 104 typically represents an oxide layer and may include any other structures of the integrated circuit. By way of example and not by way of limitation, substrate 104 may represent a gate oxide or even any oxide layer above a shallow trench isolation (STI) area. Above fuse portion 102, another oxide layer 106 is conformally deposited. A silicon nitride layer 108 is then deposited above oxide layer 106.
Above silicon nitride layer 108, a photoresist layer 110 is deposited and patterned to form an opening 112. Patterned photoresist mask 110 is then employed to etch through silicon nitride layer 108 to expose a portion of oxide layer 106 above fuse portion 102. After an opening in silicon nitride layer 108 is formed, a subsequent isotropic etch is performed to create the microcavity. As is apparent, silicon nitride layer 108 acts as a hard mask during the isotropic etch of microcavity 202.
In FIG. 2, microcavity 202 has been isotropically etched out of oxide layer 106 through the opening in silicon nitride layer 108. The microcavity etch preferably employs an etch process that is selective both to the liner material of fuse portion 102 and silicon nitride layer 108.
Subsequent to the formation of microcavity 202, a plug layer 206, e.g., another oxide layer, is then deposited. The deposition process that forms plug layer 206 is such that the opening in the silicon nitride layer is sealed with the plug material while microcavity 202 is left hollow. Thus fuse portion 102 is essentially sealed within microcavity 202 after the deposition of plug layer 206. Accordingly, any particulate material that may be formed when fuse portion 102 is blown is kept contained within microcavity 202, thereby minimizing or essentially eliminating any possibility of particulate contamination of the IC surface.
It has been found, however, that the conventional process of forming electrically blowable fuse 100 has some disadvantages. In particular, the prior art technique of forming electrically blowable fuses requires at least one photolithography step to pattern a hard mask out of silicon nitride layer 108. As is known by those skilled in the art, photolithography is an expensive process and is therefore generally undesirable from a cost standpoint. Further, as the density of the integrated circuit increases and its feature sizes decrease, accurate alignment becomes problematic. By way of example, as fuse portion 102 decreases in width and the adjacent fuses and/or devices are packed closer together, the accurate alignment of opening 112 in photoresist layer 110 with fuse portion 102 becomes increasingly difficult. These and other challenges presented by the photolithography step render the fabrication of electrically blowable fuses 100 unduly expensive and, in many cases, even prohibitively expensive.
In view of the foregoing, there are desired improved techniques for fabricating electrically blowable fuses. In particular, there are desired improved techniques for forming electrically blowable fuses that do not require the use of a photolithography step to form a hard mask for the subsequent microcavity etch.